Measurements of the bulk, C-axis electromechanical coupling constant as a function of AlN film quality

Piezoelectric thin film AIN has great potential for on-chip devices such as thin-film resonator (TFR)based bandpass filters. The AIN electromechanical coupling constant, K-2, is an important material parameter that determines the maximum possible bandwidth for bandpass filters. Using a previously published extraction technique, the bulk c-axis electromechanical coupling constant was measured as a function of the AIN x-ray diffraction rocking curve [full width at half maximum (FWHM)]. For FWHM values of less than approximately 4 degrees, K-2 saturates at approximately 6.5%, equivalent to the value for epitaxial A1N, For FWHM values >40 degrees K-2 gradually decreases to approximately 2.5% at a FWHM of 7.5 degrees. These results indicate that the maximum possible bandwidth for TFR-based bandpass filters using polycrystalline AIN is approximately 80 IL MHz and that, for 60-MHz bandwidth PCS applications, an AIN firm quality of >5.5 degrees FWHM is required.